Cryogenic arc furnace and method of forming materials

ABSTRACT

This disclosure provides apparatus for achieving rapidly a high temperature arc discharge in the region of a material to be vaporized. Surrounding the region of the arc discharge is a cryogenic fluid against which both the arc and the vaporized produces exert pressure. The effect of the presence of the cryogenic fluid adjacent to the high temperature region is to constrain the arc discharge strongly and to quench rapidly the material in the vapor state to the solid state. As a consequence of the localized heating and rapid quenching in the cryogenic arc furnace, special materials and physical states thereof are achieved. Illustratively, chemical products and amorphous conditions of materials are achieved for the practice of this disclosure not heretofore contemplated in the practice of the prior art. For an embodiment of this disclosure, the material to be vaporized is ab initio established in location for a capacitive arc discharge and the capacitor plates are caused by mechanical shock to approach each other so that the discharge occurs preferentially at a preselected path on the material. Practice of this invention is readily extrapolated to the very high temperatures required for fusion experiments in liquid deuterium, e.g., greater than 100,000*C.

United States Patent [191 Thompson 1March 13, 1973 CRYOGENIC ARC FURNACEAND METHOD OF FORMING MATERIALS [75] Inventor: William A. Thompson,Yorktown Heights, NY.

[73] Assignee: International Business Machines Corporation, Armonk, N.Y.

[22] Filed: Dec. 31, 1970 [21] Appl.N0.: 103,086

Primary Examiner F. C. Edmundson Attorney-Hanifm & Jancin and Bernard N.Wiener [57] ABSTRACT This disclosure provides apparatus for achievingrapidly a high temperature are discharge in the region of a material tobe vaporized. Surrounding the region of the arc discharge is a cryogenicfluid against which both the arc and the vaporized produces exertpressure. The effect of the presence of the cryogenic fluid adjacent tothe high temperature region is to constrain the arc discharge stronglyand to quench rapidly the material in the vapor state to the solidstate. As a consequence of the localized heating and rapid quenching inthe cryogenic arc furnace, special materials and physical states thereofare achieved. Illustratively, chemical products and amorphous conditionsof materials are achieved for the practice of this disclosure notheretofore contemplated in the practice of the prior art.

For an embodiment of this disclosure, the material to be vaporized is abinitio established in location for a capacitive arc discharge and thecapacitor plates are caused by mechanical shock to approach each otherso that the discharge occurs preferentially at a preselected path on thematerial.

Practice of this invention is readily extrapolated to the very hightemperatures required for fusion experiments in liquid deuterium, e.g.,greater than 100,000PC.

13 Claims, 3 Drawing Figures PATENTEDHARISISYS 3,720,598

SHEET 1 [IF 2 i l as 34 W 24 18 I I c INVENTOR VOLTAGE L1 22 I WILLIAMA. THOMPSON SOURCE 20 ATTORNEY CRYOGENIC ARC FURNACE AND METHOD OFFORMING MATERIALS BACKGROUND OF THE INVENTION Arc discharges are knownin the prior art for vaporizing materials at high temperatures.Cryogenic fluids have been utilized for quenching high temperatureenvironments. However, the simultaneous utilization of a plasmagenerating arc discharge in the intimate presence of a cryogenic fluidhas not been utilized for establishing chemical and physical states inmaterials although arc discharges have been used to study the characterof non-cryogenic liquids.

In co-pending application Ser. No. 15,788 filed Mar. 2, 1970 andcommonly assigned, there is provided superconducting oscillators andmethod for making the same wherein a very small Josephson oscillator isfabricated by spark erosion between capacitor elec trodes comprising thematerials to be vaporized. Spark erosion occurs in a liquid heliumenvironment which causes the formation ofa Josephson junction havingextremely small dimensions between the electrodes. Accordingly, theexplicit distinction of provisions of this invention involve thecontrolled plasma formation of material and its simultaneous quenchingthrough use of an apparatus ancillary to the material itself.

ADVANTAGES OF THE INVENTION In order to achieve relatively hightemperatures for establishing new chemical and physical states inmaterials, it has been discovered for the practice of this inventionthat use of superconducting electrodes in a cryogenic environment permitconcentration of sufficient energy in a localized region to effectsufficiently high temperature and pressure in a vaporized material whichupon the rapid quenching through cryogenic fluid within which thevaporization occurs obtains materials in states thereof not known in theprior art.

The prior art has known how to produce small circuit elements throughuse of photoresist techniques together with diffusion of ancillarymaterials into a semi-conductor material. Through the practice of thisinvention very small circuit elements of amorphous material orsuperconducting material are produced in a cryogenic environment. Thus,high temperatures states are frozen into the material which do not altersufficiently at operational temperatures of interest to change eitherchemical or physical state.

Among the advantages of this invention obtained by the practice thereof,are provision for small circuit elements on substrates, chemistry of newhigh tempera ture compounds and intermetallics, chemistry ofsuperconductor and amorphous materials, and small grain sizemetallurgical materials.

There will now be provided additional discussion of advantages of thisinvention for providing small circuit elements of wafers insemiconductor technology. The circuit elements dimension is of the orderof micron size regions. Although the precise initial portion ofelectrical discharge from the capacitor probe point may not be known, itis readily ascertained relative to the geometry of the substrate waferand thereafter is essentially fixed relative to the probe for any givenfabrication procedure. The localization of the arc discharge isdetermined both by the geometry of the capacitor probe and theconstraint on the discharge by the proximatecryogenic fluid.

SUMMARY OF THE INVENTION The practice of this invention is based on thediscovery that if superconducting leads are used throughout thedischarge circuit to a point discharge in a cryogenic fluid, a hightemperature plasma of material in the arc discharge is formed andrapidly quenched by the proximate cryogenic fluid. The rapid quench ofthe plasma provides new chemical and physical states for the materialestablished in the arc discharge and homogenized distribution of thecomponents of the material throughout the body of the resultant solidstate material. A device for the practice of this invention incorporatesa capacitor with superconducting plates and superconducting electricalenergy transfer leads thereto, a hearth in the vicinity of the resultantarc discharge for holding initially the material to be vaporized to aplasma, and a cryogenic fluid environment within which the arc dischargeis established. Since the cryogenic fluid is non-conducting, iteffectively constrains the arc but does not participate therein. Thedevice includes mechanical or electrical means for causing the capacitorprobe temporally to approach the hearth material thereby initiating thearc discharge and effecting the release of the energy stored in thecapacitor into a small spatial volume.

In greater detail, the practice of the invention provides a cryogenicarc discharge device which is capable of producing very high localtemperatures by the use of a fast superconducting discharge circuit anda fast recondensation of the vapor products by use of a cryogenic fluidin intimate contact therewith. The superconducting capacitor is chargedup and discharged through high current superconducting leads to asuperconducting point electrode which is moved vertically by amechanical shock. The use of superconductivity allows high currentdensity for discharge over a few microseconds. The use of the cryogenicfluid gives the quick recondensation of the reactant material. Thepointed discharge electrode is caused to move either over all thematerial or to lay out a predetermined circuit on the hearth substrate.

The movement of the hearth substrate under the discharge probe may beany type of mechanical manipulator providing X-Y motion, rotary motionor linear motion. However, it is required that the hearth surface beconnected electrically to the capacitor with superconducting leads.

It is an object of this invention to provide apparatus and method forobtaining rapidly relatively high temperatures in the presence of acryogenic fluid for rapidly quenching the high temperature state inducedin the material.

It is another object of this invention to utilize a capacitive arcdischarge in the presence of a material together with a cryogenic fluidadjacent thereto for rapidly vaporizing and quenching the material.

It is another object of this invention to dump a relatively large amountof energy rapidly into a small region of space and thereafter quenchimmediately the heated products by a proximate cryogenic fluid.

It is another object of this invention to provide a cryogenic arcfurnace comprising a capacitive arc discharge device having oneelectrode movable by electrical or mechanical shock so that a periodicapplication of shocks causes sequential discharges in a given region oralong a given path.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of a preferred embodiment of the invention, as illustratedin the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a functional diagram showingsome structural components of a cryogenic arc furnace for the practiceof this invention wherein an arc discharge is established between aprobe mounted on a movable capacitor plate and the discharge isrepetitively established by the synchronous cooperation of electricalmeans for charging the capacitor" and" electromechanical means fordischarging it.

FIG. 2A is a detailed cross-sectional view of a cryogenic arcdischarging apparatus embodying the principles illustrated in FIG. 1 andshowing the cooperative environment of capacitive arc discharge region,cryogenic fluid, means for charging the capacitor and means fordischarging it to modify either a chemical or physical state in thematerial of the arc discharge by rapidly vaporizing it andsimultaneously quenching it via the cryogenic fluid.

FIG. 2B illustrates a modification of FIG. 2A wherein the support mountfor the hearth is indicated to be also movable in the plane relative tothe capacitor probe as well as vertically with respect to it.

EMBODIMENTS OF THE INVENTION Exemplary embodiments of this invention areillustrated in FIGS. 1A, 2A and 2B of which FIG. 1 is a functionaldiagram showing the correspondence of the several material elements ofthe apparatus, FIG. 2A is a derived cross-sectional drawing showing theactual relationship of the several aspects of the apparatus in anoperational environment and FIG. 2B is an addendum to FIG. 2A forindicating means for adding planar movement of the material mountingplatform relative to the capacitive point electrode.

With reference to FIG. 1, a capacitor having plates 10 and 18 isconnected to electrodes 20 and 22, respectively, of the arc dischargefurnace. The electrodes 20 and 22 are respectively the locations for thematerial to be treated and the determining point for the arc discharge.A direct voltage source 34 charges the capacitor via a resistor 36. Thedischarge of the capacitor is achieved by shock from plunger 24 drivenby magnetic coil 26 which is powered from alternating current source 32.The capacitor electrodes and the mechanical shock device are within thechamber of a cryogenic fluid dewar 30.

The embodiment presented in detailed cross-sectional view in FIG. 2Aincludes a capacitor having a base plate 10 and a top plate 12.Superconducting superconductors such as tantalum or niobium are used forthe base plate 10 and the top plate 12. Base plate 10 and top plate 12together comprise one electrode of the capacitor. Tantalum is especiallysuitable as the capacitor material because of the beneficialcharacteristics of the tantalum oxide, i.e., thinness and pin hole free.Further, the tantalum oxide is readily grown naturally on the tantalumsurface in a high temperature oxidizing atmosphere. The other electrodeof the capacitor 18 is a thin tantalum sheet having a natural oxide oranodized oxide layer thereon. lllustratively the capacitor plate 18 is athin membrane of tantalum of about 10 mils thickness. In principle, thelinear dimension of the capacitor is limited by the parameters of thecryogenic environment in which it is inserted and the amount of requiredcapacitance. The central electrode 18 of the capacitor and the two outerelectrodes 10 and 12 comprise a transmission strip line which transmitsthe stored energy in the capacitor to the arc discharge at electrode 22.The hearth 20 is shown in screw form for establishing the material to bevaporized in the cryogenic arc furnace. The material 23 is establishedin the hearth adjacent to the point electrode 22. Although the hearth 20need not be a superconducting material, it is beneficial so as tominimize resistive losses in the arc discharge. According to the designof the apparatus shown in FIG. 2A the screw 20 is positioned verticallyin a manner which will be described hereinafter.

The discharge point electrode 22 is also a superconductor such astantalum which is affixed to the capacitor membrane 18. The dischargeelectrode 22 is welded or established in good electrical contact forconventional fabrication technique to the surface of the capacitormembrane 18. The shape of the point electrode 22 is determined by thesize of the discharge area to be formed on the surface of the hearth 20.

The impact slug is mounted in upper housing portion 12A establishedabove discharge point electrode 22. Drive coil 26 surrounds impact slug24. The drive coil 26 is connected to oscillator 22 which periodicallydrives the impact slug 24 against the membrane 18 causing pointelectrode 22 to approach the hearth 20. The timing of the alternatingvoltage is coordinated with the mechanical recovery of the systemcomprising the capacitive membrane 18, point electrode 22 and impactslug 24. The impact slug 24 is insulated from the capacitor 12 by aninsulation sleeve 28 which may be, for example, of teflon or othersuitable dielectric, providing insulation properties for the operationof the apparatus of FIG. 2A. The electrical circuit for charging thecapacitor comprises a battery 34 and resistor 36 connected by leads 35Aand 358 to capacitor base plate 10. and capacitor membrane 18, it beingunderstood that capacitor top plate 12 is electrically connected to baseplate 10 and is therefore at the same electrical potential.

The cryogenic dewar 30 surrounds the capacitor and the drive mechanismand chamber 31 thereof is filled with cryogenic fluid, e.g., He,introduced into the dewar chamber 31 via entrance orifice 31A andremoved therefrom via exit orifice 318. Base plate 10 has orifices 40from dewar chamber 31 into chamber 41 wherein .are located the pointelectrode 22 and hearth 20 with the material 23.

The capacitor leads must remain superconducting at the current load,e.g., 10 to 10 amps/cm. Probe superconductors and alloys mayillustratively be Nb, Ta, Pb, NbTa, NbZr.

The reactants are placed in the electrode hearth 20 either as smallgrains or as evaporated layers. A mechanical adjustment is made to setthe electrodeelectrode distance both before and after impact by slug 24.The'discharge spacing is set by varying the coil 26 drive power and thepredischarge spacing is set by the hearth adjustment to be low currentwith field emission insufficient to cause heating or capacitordischarge, e.g., to 100,000A. The capacitor is then charged to testvoltage and the impact slug 24 is driven down onto the drumhead 18surface giving sharp vertical motion to the probe 22 and discharging thecapacitor into the arc plasma formed immediately above the surface ofhearth 20. The probe is tantalum with an oxide coating. An exemplary rigusing a slow (millisecond heating) 0.01 joule discharge obtainedtemperature greater than 1,300C.

Further details of the nature of the materials and the operation of thecryogenic arc furnace illustrated in FIGS. 1, 2A and 2B will now bepresented. In practice the superconducting capacitor is charged toseveral hundred volts which is a few joules and then discharged throughthe high current leads, i.e., the positive leads 10 and 12 and thenegative lead 18 of FIG. 2A. Thus, a high current density is achieved inthe discharge for a few microseconds. The cryogenic fluid, e.g., liquidHe, in the chamber 31 is introduced into the chamber 41 in the presenceof the hearth material and the capacitor discharge electrodes viaorifices 40. The liquid helium in the chamber 41 causes the reactiveproducts to recondense quickly, thereby freezing in the high temperaturephases. Additionally, because of the mechanical vacuum properties ofliquid helium, contamination of the reacted product is minimized.

The discharge time of the device is determined by the electrical circuitcharacteristics in the cryogenic environment. Important parameters arethe superconducting electrode path between the capacitive region whichis on the outside of the diaphragm to the central discharge point whichis at the center. This is a superconducting waveguide with low losses upto discharge times being of the order of the energy gap frequency. Thedischarge may be increased to the gap frequency which is in the highmicrowave frequency region. Other than that the discharge time isdetermined solely by the geometrical waveguide properties of thecapacitor. The charging time of the capacitor is determined by the RCtime constant of the electrical circuit consisting of the externalbattery 34 and resistor 36 combination in series with the capacitorwhich must be fast enough to recharge the capacitor before the nextdischarge. Since the discharge time (frequency) is in the order ofmilliseconds, the external charging time must be faster than this. Underhigh frequency discharges and large energy discharge of certainoperational conditions the pressure in the discharge chamber may besufficient to require relief of the pressure by additional vent holes oruse ofa pump liquid system.

FIG. 28 illustrates a modification of the device as shown in FIG. 2Awhereby the hearth screw 20, hearth 20A and material therein 23 isreadily moved via seal 21 forpositioning any point thereof bothvertically and horizontally relative to the probe point electrode 22.The hearth screw 20 is shown expanded in size so that it mayconveniently support a conventional X-Y adjustment table relative to theprobe point electrode 22. The X-Y table is indicated generally by thearrow 50. The X-Y table and the controls therefor consist of X movementstation 52 and Y movement station 54. The hearth 20A and material 23therein is supported on Y movement stage 54. Control cable 56 isconnected to the X stage 52 and via the hearth shaft 58 is conveyed tothe knob 60 whose rotation controls the X stage movement. The Y stage 54is controlled by cable 62 which communicates via hearth shaft 58 to knob64 whose rotation controls the Y movement. The Z movement is controlledby rotation of the hearth screw 20 as is accomplished in the design ofFIG. 2A.

EXAMPLES OF THE INVENTION Examples of this invention in accordance withthe principles thereof will now be presented for the systems GaAstransformed to Ga and La Se transformed to La Se The gallium arsenidewafers doped with zinc with a carrier concentration of 2 X 10 carriersper cc were used as starting wafers in the device described. Theformation temperature was in the range from 1.35 to 4.2, the boilingpoint of liquid helium. The discharge voltage was normally in the rangeof l 10 volts on the capacitor providing a temperature greater thanl,300C. Tunneling curves of current vs. voltage were obtained bothbefore and after the formation of amorphous gallium (Ga) from a (GaAs)wafer. The tunneling trace before amorphous Ga formation showed thecharacteristic Schottky barrier type tunneling, the tunneling traceafter formation of the amorphous gallium on one of the wafers showed theexistence of the superconducting region. The superconducting energy gapand the transition temperature of this region was determined by thetunneling method. The superconducting transition temperature T was 8.2,in good agreement with the published data on amorphous gallium. Thesuperconducting energy gap ratio to T was 4.2, also in agreement withpublished data on amorphous gallium indicating that amorphous galliumhad formed in a small region on the wafer. The magnetic fieldcharacteristics indicated that the particle size of amorphous galliumcreated was somewhat less than 1,000A in diameter.

The lanthanum selenide system was also investigated by the technique ofthis invention. The selenium rich phase, La,Se was converted to thelanthanum rich phase, La Se, by means of vaporizing in situ and fastrecondense according to the principles of this invention. The materialproperties were investigated by electron probe tunneling. The currentvs. voltage curves indicated the enhancement of superconductivity. Thelanthanum rich phase is a good superconductor with a transitiontemperature which depends on the density of vacancies in the material.The vaporization temperature was greater than 2,000C and the formationtemperature was in the l.35 to 4.2"K range.

With the high temperatures possible through the practice of thisinvention the reactant material is made much more reactive than normallybecause it is ionized more and because there is more impact energy percollision. For example, the chemically inert element argon has been madein the prior art to form compounds by first heating it to hightemperatures in an accelerator. It is believed that new compounds of thehigh melting point elements W, Os, Rh, Mo and Ta will be formed by therapid quench technique of this invention.

The transition temperature of superconducting material is sensitive tothe crystal phase and in the search for higher temperaturesuperconductors, the high temperature crystal and amorphous phases ofthe binary, ternary and extended compositions of the presently knownsuperconductors can be looked at. Namely, through using a hearthmaterial which consists of various combinations of Nb, Si, Sn, Al, V,Ga, Ge and Zr, there may be provided unique superconductor materials.

With the pin point placement of the discharge according to theprinciples of this invention, a small region on a gallium arsenide wafercan be changed in its surface chemical state, e.g., 4GaAs+3O 2Ga O +A 5or superconducting electrical elements of amorphous Ga, e.g., GaAsGa+As, can be placed at preferred locations.

Superconductors with higher transition temperatures may be obtained bygoing to high temperature formation through the practice of thisinvention. It is known that amorphous material has lower phonon energyvalues than the corresponding crystalline material which enhance thesuperconducting correlations in the material thereby giving it a highertransition temperature. Examples are the crystalline gallium which has atransition temperature of a little over 1 and the amorphous galliumwhich has a transition temperature of 8.4K, an enhancement of about 7times.

I claim:

1. Apparatus for changing a material from one state to a different statecomprising:

means for establishing a cryogenic fluid in a region;

means for establishing an electrical discharge circuit in said cryogenicfluid and for defining a path for said discharge in said region;

means for supplying electrical energy to said electrical dischargecircuit;

means for establishing a material to be vaporized in said path; and

means for discharging said electrical discharge circuit for establishingsaid discharge in said path. 2. Apparatus as set forth in claim 1wherein said means for discharging said electrical discharge circuitincludes mechanical means.

3. Apparatus according to claim 2 wherein said mechanical meansdischarges said electrical discharge circuit periodically.

4. Apparatus according to claim 3 wherein said periodicity of saiddischarging is slower than the charging time for said electricaldischarge circuit.

5. Apparatus for changing a material from one state to a different statecomprising:

a capacitor having a first and second plate positioned on either side ofsaid material to be changed;

means for charging the plates of said capacitor sufficient to causevaporization of said material upon discharge;

means for discharging said capacitor; and

means for establishing the discharge circuit of said capacitor at acryogenic temperature to rapidly quench said vaporized material andthereby to establish said material in a different state.

6. Apparatus of claim 5 wherein the plates of said capacitor are made ofmaterials capable of acting as superconductors.

7. Apparatus of claim 5 wherein the discharge circuit of said capacitoris established by surrounding such circuit with a cryogenic fluid.

8. Apparatus for changing a material from one state to another statecomprising:

a capacitor structure including two capacitor plates consisting ofrespective superconductive materials;

a hearth for material whose state is to be changed mounted on one saidplate;

a probe point on said other capacitor plate juxtaposed relative to saidhearth;

mechanical means for moving temporally said probe relative to saidhearth to discharge said capacitor; means for establishing a cryogenicfluid in said are discharge region;

and electrical means for charging said capacitor with energy for saidare discharge.

9. Apparatus as set forth in claim 8 wherein said one capacitor plateincludes two portions, one said portion mounting said hearth and saidother portion mounting said mechanical means;

' said portion mounting said hearth including a means for juxtaposingsaid hearth relative to said probe point;

said other capacitor plate being in membrane form;

said mechanical means including:

electromagnetic coil means mounted on said other portion of saidcapacitor for mechanically driving said probe point to cause it tochange relative position to said hearth;

an impact slug in said electromagnetic coil means;

and

alternating current electrical means for driving repetitively saidimpact slug on said superconductive capacitor membrane synchronouslywith the mechanical vibration parameter thereof.

10. Apparatus for generating a plasma comprising:

means for establishing a cryogenic fluid in a region;

means for establishing an electrical discharge circuit in said cryogenicfluid and for defining a path for said discharge in said region;

means for supplying electrical energy to said electrical dischargecircuit; and

means for discharging said electrical discharge circuit for establishingsaid discharge in said path.

11. Apparatus asset forth in claim 10 wherein said means for dischargingsaid electrical discharge circuit includes mechanical means.

12. Apparatus according to claim 11 wherein said mechanical meansdischarges said electrical discharge circuit periodically.

13. Apparatus according to claim 12 wherein said periodicity of saiddischarging is slower than the charging time for said electricaldischarge circuit.

a m a a a

1. Apparatus for changing a material from one state to a different statecomprising: means for establishing a cryogenic fluid in a region; meansfor establishing an electrical discharge circuit in said cryogenic fluidand for defining a path for said discharge in said region; means forsupplying electrical energy to said electrical discharge circuit; meansfor establishing a material to be vaporized in said path; and means fordischarging said electrical discharge circuit for establishing Saiddischarge in said path.
 2. Apparatus as set forth in claim 1 whereinsaid means for discharging said electrical discharge circuit includesmechanical means.
 3. Apparatus according to claim 2 wherein saidmechanical means discharges said electrical discharge circuitperiodically.
 4. Apparatus according to claim 3 wherein said periodicityof said discharging is slower than the charging time for said electricaldischarge circuit.
 5. Apparatus for changing a material from one stateto a different state comprising: a capacitor having a first and secondplate positioned on either side of said material to be changed; meansfor charging the plates of said capacitor sufficient to causevaporization of said material upon discharge; means for discharging saidcapacitor; and means for establishing the discharge circuit of saidcapacitor at a cryogenic temperature to rapidly quench said vaporizedmaterial and thereby to establish said material in a different state. 6.Apparatus of claim 5 wherein the plates of said capacitor are made ofmaterials capable of acting as superconductors.
 7. Apparatus of claim 5wherein the discharge circuit of said capacitor is established bysurrounding such circuit with a cryogenic fluid.
 8. Apparatus forchanging a material from one state to another state comprising: acapacitor structure including two capacitor plates consisting ofrespective superconductive materials; a hearth for material whose stateis to be changed mounted on one said plate; a probe point on said othercapacitor plate juxtaposed relative to said hearth; mechanical means formoving temporally said probe relative to said hearth to discharge saidcapacitor; means for establishing a cryogenic fluid in said arcdischarge region; and electrical means for charging said capacitor withenergy for said arc discharge.
 9. Apparatus as set forth in claim 8wherein said one capacitor plate includes two portions, one said portionmounting said hearth and said other portion mounting said mechanicalmeans; said portion mounting said hearth including a means forjuxtaposing said hearth relative to said probe point; said othercapacitor plate being in membrane form; said mechanical means including:electromagnetic coil means mounted on said other portion of saidcapacitor for mechanically driving said probe point to cause it tochange relative position to said hearth; an impact slug in saidelectromagnetic coil means; and alternating current electrical means fordriving repetitively said impact slug on said superconductive capacitormembrane synchronously with the mechanical vibration parameter thereof.10. Apparatus for generating a plasma comprising: means for establishinga cryogenic fluid in a region; means for establishing an electricaldischarge circuit in said cryogenic fluid and for defining a path forsaid discharge in said region; means for supplying electrical energy tosaid electrical discharge circuit; and means for discharging saidelectrical discharge circuit for establishing said discharge in saidpath.
 11. Apparatus as set forth in claim 10 wherein said means fordischarging said electrical discharge circuit includes mechanical means.12. Apparatus according to claim 11 wherein said mechanical meansdischarges said electrical discharge circuit periodically.